Particular matter (PM), NOx (NO or NO2), etc. which may have a risk of affecting the environment are contained in exhaust gas discharged from an internal combustion engine such as a diesel engine or the like. An exhaust gas purification system having an NOx catalyst disposed in an exhaust gas passage is known as an exhaust gas purification system used to purify NOx out of the above materials.
An exhaust gas purification system using NOx storage catalyst and an SCR (Selective Catalytic Reduction) system using selective reduction catalyst are known as such an exhaust gas purification system. Accordingly to the exhaust gas purification system using the NOx storage catalyst, NOx in exhaust gas is absorbed under the state that the air-fuel ratio of the exhaust gas is under a lean state, and when the air-fuel ratio of the exhaust gas is changed to a rich state, NOx is subjected to reductive reaction with hydro carbon (HC) and carbon oxide (CO) in the exhaust gas while NOx is discharged, thereby purifying the exhaust gas. Furthermore, the SCR system uses catalyst for selectively reducing NOx in exhaust gas and supplies reducing agent mainly containing urea or HC into the exhaust gas so that NOx is subjected to reductive reaction with the catalyst, thereby purifying the exhaust gas.
In these exhaust gas purification systems, an NOx sensor is normally disposed in an exhaust gas passage at the upstream side or downstream side of an NOx catalyst, and used to determine an injection amount of reducing agent and control the operation state of an internal combustion engine so that purification of NOx is efficiently performed. Furthermore, the NOx sensor may be used to diagnose whether the exhaust gas purification system operates normally or not. Accordingly, high reliability is required to the output of the NOx sensor.
Therefore, there has been proposed a device for determining abnormality of an NOx sensor used in an exhaust gas purification system.
For example, an NOx sensor abnormality determining device for determining abnormality of an NOx sensor which implements a flow shown in FIG. 10 and performs proper reproduction of NOx storage catalyst have been disclosed (see Patent Document 1). The NOx sensor is provided at the exhaust gas downstream side of NOx storage catalyst, and the NOx sensor abnormality determining device comprises NOx discharging means for forcedly generating a state that NOx in the NOx storage catalyst is discharged, real NOx reduction rate setting means for calculating an actual NOx reduction rate of the NOx storage catalyst under the state that NOx is discharged by the NOx discharging means, reference NOx reduction rate setting means for presetting a reference NOx reduction rate on the basis of the operation state of an engine, and abnormality determining means for comparing the actual NOx reduction rate with the reference NOx reduction rate to determine abnormality of the NOx sensor.
Furthermore, an air-fuel ratio sensor, a gas sensor, etc. are further known as sensors provided in the exhaust gas passage of the internal combustion engine, and methods of diagnosing deterioration of these sensors have been also proposed.
For example, there has been proposed a deterioration diagnosis device for an air-fuel ratio sensor in which preliminary diagnosis is performed under air-fuel ratio feedback control to implement a flow shown in FIG. 11, and deterioration diagnosis following forced vibration is performed in accordance with the preliminary diagnosis result. More specifically, there has been disclosed a deterioration diagnosis device comprising an air-fuel ratio sensor for detecting the air-fuel ratio of air-fuel mixture, air-fuel ratio control means for executing feedback control on the air-fuel ratio of the air-fuel mixture on the basis of a detection signal of the air-fuel ratio sensor, detection capability determining means for determining detection capability of the air-fuel ratio sensor on the basis of the detection signal of the air-fuel ratio sensor, forced variation means for forcedly varying the air-fuel ratio of the air-fuel mixture in preference to the air-fuel ratio control means under a predetermined condition, and deterioration determining means for determining deterioration of the air-fuel ratio sensor on the basis of the detection signal of the air-fuel ratio sensor when the forced variation is performed by the forced variation means (see Patent Document 2).    Patent Document 1: JP-A-2004-270468 (Scope of claim for Patent)    Patent Document 2: Patent Number JP 3134698 (Scope of claim for Patent)